This invention relates generally to multilayer capacitor chips. More particularly, this invention relates to multilayer ceramic capacitors having a high dielectric constant and which are relatively uniform with respect to changing temperature.
It is well known that typical ceramic dielectrics (such as Class II dielectrics as defined by the Electronic Industries Association, EIA) have dielectric constants which undergo change with respect to temperature. Typically, the temperature stability or TC curve is bell shaped. A typical TC curve for such a prior art ceramic material is shown in FIG. 1. It is clear from a review of FIG. 1 that the dielectric constant (and capacitance) of a ceramic material varies greatly with temperature. The temperature at which the dielectric constant is a maximum is termed the curie point.
As a result of the phenomenon associated with known ceramic materials illustrated in FIG. 1, ceramic capacitor manufacturers have attempted to develop ceramic dielectric materials with both higher dielectric constants and better temperature stability characteristics. Unfortunately, this approach has been difficult since the present dielectric materials used in multilayer ceramic chip capacitors (for any given layer) inherently have two mutually exclusive characteristics including either good temperature stability and relatively low capacitance (X7R); or relatively poor temperature stability and very high capacitance (Z5U). A ceramic dielectric material having both uniform temperature stability and high dielectric constant is presently not known to applicant.
The ceramic industry has labeled relatively stable temperature characteristic curves for ceramics as X7R type ceramics. One common method of achieving X7R temperature stable curves is to combine a high purity ceramic such as Barium Titanate with a number of known dopants.
Unfortunately, this method suffers from certain drawbacks. For example, modifying pure Barium Titanate with dopants to achieve better temperature stability characteristics leads to lower dielectric constants than are available with the unmodified Barium Titanate.
In many applications, it has become increasingly important and desirable to have the temperature stability of an X7R material, but with a much higher dielectric constant such as found in a Z5U dielectric. Such applications include decoupling of the newer highly dense memory integrated circuits and other VLSI devices which handle high currents at high switching speeds. Still other applications include decoupling in oil drilling equipment, aerospace and military applications where temperature stability is vital.